Cancer is a multi-step disease involving a series of genetic alterations that result in dysregulation of cell proliferation and differentiation. These sporadic mutations, some of which are caused by factors such as environmental chemicals and radiation, lead to the activation of oncogenes, the loss of tumor suppression and the activation of programmed cell death. Our laboratory has been involved for many years in studies dealing with the molecular biology of the ets gene family members. Having been the first group to demonstrate that these genes exist as a family, we cloned and characterized the first several family members, including ETS1, ETS2, ERG, ERGB/Fli1, and ERF. While it has been shown that ETS1 is an oncogene when it is transduced by the E26 virus, we have determined the role of many of ets genes in cell proliferation, normal differentiation, and in some cases abnormal development leading to cancer. The experiments described in this proposal are designed to understand in detail the mechanism by which an isotype of ETS1, generated as a splicing variant and referred to as p42-ETS1, represses tumorigenicity and induces apoptosis in epithelial cells. The potentially important role of the p42- ETS1 protein in the maintenance of normal T-cell populations will also be examined. To accomplish these goals we will, i) analyze the molecular mechanism by which the p42-ETS1 protein leads to the induction of apoptosis in epithelial cancer cells, ii) determine which are the critical DNA regions (domains) within the p42-ETS1 gene that are required for the ability of the gene product to induce apoptosis in epithelial cancer cells and in T-cells, iii) identify and characterize the function of p42-ETS1 target genes involved in the induction of apoptosis in epithelial cancer cells, using a tetracycline inducible system, and iv) study the normal function of the p42-ETS1 in T-cells by generating knock-out mice individually lacking one of the ETS1 isotypes. This work can have profound medical significance in that it may open new insights as to the potential role of the p42-ETS1 splice variant ETS1 isotype in controlling cell proliferation by the induction of apoptosis in epithelial cell cancers. Such insights may provide a basis for the design of a gene therapy approach utilizing the ability of p42-ETS1 to initiate cell death in cancer cells.